1. Field of the Invention
The present invention relates to a method of evaluating illumination distribution, a manufacturing method of an optical member, an illumination optical device, an exposure apparatus, and an exposure method, and particularly to manufacturing of a micro fly's-eye lens used in an illumination optical apparatus of an exposure apparatus for manufacturing micro devices, e.g. semiconductor devices, imaging devices, liquid crystal display devices, thin film magnetic heads or the like, through lithography processing.
2. Related Background Art
In such a typical exposure apparatus, a light beam emitted from a light source enters a fly's-eye lens to form, on its back focal plane, a secondary light source which is composed of numerous light sources. After having been limited by means of an aperture diaphragm disposed in proximity to the back focal plane of the fly's-eye lens, the light beam from the secondary light source enters a condenser lens. The aperture diaphragm limits the shape or size of the secondary light source to a desired shape or size, depending on a desired illumination condition (exposure condition).
The light beam collected by the condenser lens super-imposingly illuminates a mask having a predefined pattern formed thereon. The light which transmitted through the mask pattern forms an image on a wafer by means of projection optics. In such a manner, a mask pattern is projection-exposed (transferred) on the wafer. Here, the pattern formed on the mask is highly integrated, whereby it is essential to have a uniform illumination distribution on the wafer in order to precisely transfer this micro-pattern on the wafer.
In an exposure apparatus having the above-mentioned arrangement, it is necessary to provide as many as possible micro lens elements which compose the fly's-eye lens in order to enhance the uniformity of the illumination distribution. It is also necessary to form a secondary light source having a shape similar to the desired shape in order to avoid optical loss at the aperture diaphragm. Accordingly, it is conceivable, for example, to make the size of the micro lens elements composing the fly's-eye lens very small, in other words, to use a micro fly's-eye lens.
Thus, a micro fly's-eye lens is composed by monolithically forming numerous micro refracting surfaces whereas a fly's-eye lens is composed by arranging numerous lens elements in rows and columns and closely packed. That is, whereas the fly's-eye lens is composed by combining numerous lens elements which have been individually polished into a dense array, the micro fly's-eye lens is composed by forming numerous micro refracting surfaces by applying MEMS technique (such as lithography and etching), for example, to a parallel plane glass plate.
Thus, in manufacturing, the fly's-eye lens can be assembled by inspecting the shape of polished refracting surface lens elements, selecting a lens element which satisfy the specification, and using only the lens element having a refracting surface with a high precision. However, in a micro fly's-eye lens, since all of the micro refracting surfaces should be manufactured simultaneously by etching, which is difficult to obtain a surface shape of a higher quality than the polishing, the yield rate becomes considerably lower than that of a fly's-eye lens.
In the micro fly's-eye lens currently under development, shape precision (precision of the surface shape of micro refracting surface) with an order of several tens of nanometer (nm) is required when designing. For example, according to a typical exemplary design, illumination variation (illumination unevenness) of 0.1% occurs due to a 10 nm shape variation (shape error) of the micro refracting surface, in the case where all of the micro refracting surfaces within the micro fly's-eye lens have a shape identical to one another. Therefore, very high processing precision of several tens of nm is required within the effective region of the micro fly's-eye lens in order to achieve the desired specification (spec) to suppress the illumination unevenness on the field of illumination below 0.5%.
However, in a micro fly's-eye lens with all of the micro refracting surfaces being simultaneously manufactured by etching, it is difficult to process all of the micro refracting surfaces with a very high precision of several tens of nm. Thus, as a method to relax the requirement for such high shape precision, it is conceivable to moderately vary and average the shape of micro refracting surfaces within the micro fly's-eye lens. In other words, the requirement for the shape precision of micro refracting surfaces can be significantly relaxed by moderately changing (moderately varying) the shape of micro refracting surfaces within the effective region of the micro fly's-eye lens.
When applying a method which relaxes the requirement for the shape precision by moderately changing the shape of micro refracting surfaces, it is important to control the non-uniformity of the shape of micro refracting surfaces within a given range in order to stably mass-manufacture micro fly's-eye lenses. To control the shape non-uniformity of the micro refracting surfaces, a method which measures the surface shape of each micro refracting surface using a shape measurement apparatus, for example, can be used. However, this method has an inconvenience in that it not only requires troublesome work in predicting the illumination distribution by calculation from shape data of each micro refracting surface, but also is low in precision of the predicted illumination distribution.
On the other hand, it is conceivable to employ a method (hereafter referred to as “HITS”) which measures the illumination distribution generated by means of a plurality of partial regions within the effective region of the micro fly's-eye lens, using probe light with beam size substantially smaller than the effective region of the micro fly's-eye lens. When controlling shape non-uniformity of the micro refracting surfaces by HITS method, although the troublesome work of predicting the illumination distribution by calculation from the shape data of each micro refracting surface becomes unnecessary, a need arises to analytically evaluate the illumination unevenness component by expressing, according to a simple function, the illumination distribution data (raw data) obtained by measuring the illumination distribution generated by means of respective partial regions.